If you want to transfer a call to a colleague to speak to them to make sure they are available to take the call you can do it as follows:

1. When on a call that needs to be transferred, press the Consultation key as shown below: 

2. This will put the person on hold and open the "consultation" call.

3. Enter the colleagues number and press the Dial key.

4. If they are available, press the Transfer key to bridge the calls together as shown below: 

5. If they are not available, press the Consultation key again (shown below) in order to resume the initial call.

6. This will take you back to the person you were speaking with.